Basically, an axial mechanical seal includes a pair of flat sealing faces forming part of a rotating seat ring and a stationary sealing ring; these flat faces are urged into fluid sealing engagement with one another by the application of force (usually springs) to one or both of them. In addition, these faces are subjected to the pressurized fluid contained within a portion of the seal housing into which a rotatable shaft extends.
Vibrations which are transmitted from the machine to the seal cause difficulties, especially in large units such as turbines, sterntubes, etc. Depending on the frequency and the amplitude of these vibrations, there occurs periodical lift-off of the sealing faces resulting in significant leakage therebetween. In extreme cases, this effect can cause very short suction and discharge periods in the sealing gap with high speed leakage flow and extreme high pressure differences leading to erosion of the sealing surfaces and rapid damage of the seal. These vibrations may be divided into radial and axial movements. Radial movements lead to the sliding of the sealing ring and the seat ring in the radial direction of the faces of each other; however, these movements are, in most cases, accepted by standard axial mechanical seals. On the other hand, axial movements generate forces in the area of the seals which may be divided into mass forces and hydraulic forces. In order to reduce mass forces, parts subject to such forces are usually made as light as possible. Also, by adjusting the load of the springs urging the faces in contact with one another, the area of resonance can be avoided.
The hydraulic forces are significantly larger than the mass forces as considerable amounts of fluid must be displaced and suctioned within very short time intervals. Because of the above-mentioned periodical lift-off, these hydraulic forces may lead to a constant opening of the seal faces as the forces in the springs are not capable of restoring the contact of the sealing faces in the short time intervals. This condition leads to a constant and very high leakage, eliminating the advantage of low leakage and high life expectancy of axial mechanical seals as compared to other types of seals.